1. Field of the Invention
The present invention relates generally to flowmeters, and particularly to acoustic flowmeters. More particularly still, it relates to acoustic flowmeters based on the measurement of propagation times across the whole river, and which are suitable for measuring the average flow velocity in rivers, or the like bodies of water, in which the water level varies, and also under ice.
2. Prior Art of the Invention
Supersonic flow meters are being used successfully to determine the flow velocity of water volume in rivers and canals. The mean velocity of water in the reference plane of the acoustic path is determined by measuring the propagation times of acoustic pulses in opposite directions. In order to determine the mean velocity for the entire flow cross-section, a correction factor (k) must be applied. k depends on the position of the acoustic transducers, the flow cross-section profile, the relative heights of the acoustic path, and the bottom roughness. It is known that if the acoustic path is located about 40% above the bottom, then the correction factor becomes independent of the bottom roughness and equal to unity. 40% above bottom is therefore a preferred depth of installation. However, as the water level rises, and the relative installation level above ground falls to 20% or less, the effects of the bottom contour become more significant and the uncertainty in the value of the proper correction factor k increases. The latter may be estimated by computation using uncertain assumptions about bottom contour variations upstream as well as about river width, which is not satisfactory.
The problem is further aggravated in winter, when k is also influenced by the surface contour of the ice cover, and more uncertain assumptions about the same must be made in order to determine the new k. Errors in flow velocity determination may, therefore, reach or exceed 40% in winter, because the logarithmic vertical velocity distributions, which are stable in the summer with good accuracy, are no longer stable in the winter under ice.
In order to mitigate the above mentioned deficiencies, it has been suggested that acoustic flowmeters be stacked vertically to measure at several planes, thereby apprehending the different vertical distribution of flow under ice . Such an arrangement is also likely to improve accuracy in the summer. Practical experience, however, shows that the complexity of such installation is significant, and that also as a result the susceptibility to failure is increased, the complexity of apparatus increases proportionately with the number of planes, while the improvement in accuracy remains marginal.
A further problem with multi-level installations is that a top level installation close to the ice layer (for an accurate assessment of the vertical velocity distribution) is incompatible with other theoretical requirements, for in order to avoid multi-path echoes, it is necessary to measure at minimum depth, which grows as the acoustic path length does. Thus, the top installation is constrained downwardly from above, while the bottom installation is constrained upwardly from below. These constraints sometimes cause both top and bottom installations to coincide, which means there is no room for multi-level measurement. A novel solution is therefore indicated.
The use of acoustic energy to measure flow velocity is the subject of three United States patents of general interest.
U.S. Pat. No. 3,633,415 granted Jan. 11, 1972 to Lu is entitled Flowmeter. It teaches how flow velocity is measured in any direction in the plane by placing transducers at the corners of a triangle in the plane and measuring time differences of travel of acoustic energy between transducers.
U.S. Pat. No. 3,861,211 granted Jan. 21, 1975 to Dewan is entitled Ultra-Low Flow Velocity Current Meter. This invention relates to the use of an acoustic signal to determine current velocity of water. The device makes-use of a centrally single positioned transducer with four equispaced receivers equidistant from the transducer. A carrier frequency with two different modulator frequencies is directed to the transducers and the receivers receive the transmitted pulses through the water current. The velocity of the water is determined by measuring the phase angle of the different signals that pass through the water and which are received by the receivers. The different signals permits one to determine the velocity of the water.
In U.S. Pat. No. 3,949,605 granted Apr. 13, 1975 to Stallworth et al., both acoustic and electrical signals are used to measure flow in large bodies of water by comparing the delay between acoustic and electrical propagation.